Technical Field
Exemplary embodiments of the present invention relate to a belt device including an endless belt supported by a plurality of support rollers, and to an image forming apparatus such as a printer, a facsimile machine, or a copier, including the belt device.
Description of the Related Art
Color image forming apparatuses are known in which toner images of respective colors formed on a latent image carrier are primarily transferred to an intermediate transfer body in a primary transfer process, and secondarily transferred onto a sheet member in a secondary transfer process.
A secondary transfer unit of the above type of apparatus employs either a roller transfer method or a belt transfer method. In the roller transfer method, the sheet member is conveyed while being sandwiched by the intermediate transfer body and a transfer roller disposed in the secondary transfer unit, during which the secondary transfer is performed. On the other hand, in the belt transfer method, the sheet member is conveyed while being sandwiched by an endless belt (i.e., a secondary transfer belt) entrained around support rollers of the secondary transfer unit and the intermediate transfer body, during which the secondary transfer is performed.
In the belt transfer method, the sheet member is adsorbed to and conveyed at at least one portion of the secondary transfer belt upstream of the sheet conveyance direction or downstream thereof where the sheet member is sandwiched between the intermediate transfer body and the belt member (or a so-called secondary transfer nip). As a result, the sheet member is held and applied with a conveyance force not only at the secondary transfer nip where the sheet member is sandwiched between the intermediate transfer body and the belt member but also upstream and downstream in the sheet conveyance direction. Accordingly, the belt transfer method is preferable to the roller transfer method for its stable conveyance of the sheet member.
However, in the belt transfer method employing the belt, the secondary transfer belt sometimes walks to one side of the belt in the width direction (that is, belt walking), or the belt repeatedly walks to one side or the other reciprocally (that is, belt walk) similarly to any common belt conveyance device. The belt walking including wobbling occurs due to dimensional tolerances arising in assembling devices forming the secondary transfer unit, including misalignment of the rotary shaft of the plurality of support rollers around which the secondary transfer belt is entrained, variations in an outer diameter of each roller, uneven tension due to changes in the perimeter length of the secondary transfer belt itself, and the like. More specifically, when the secondary transfer belt does not rotate linearly due to those reasons and rotates with the support roller shaft deviated in the belt width direction, the belt walks toward the deviated direction.
Conventionally, various belt walk prevention means have been proposed to restrict the belt widthwise moving range within a predetermined range.
In one approach, a steering roller (or a support roller) around which the belt is entrained, inclines upon detection of the belt walking, so that the belt is moved in a direction that corrects the belt walking, which is a so-called shaft leaning device. The above approach includes a sensor to detect belt walk, and controls the inclination of the steering roller based on the detection result of the sensor.
In another approach, without using a sensor, an inclinable belt walk correction roll (or a support roller), around which the belt is entrained, is disposed. By inclining a rotary shaft of the belt walk correction roll, the belt is moved in a direction that corrects the belt walking. The belt walk correction roll includes a belt walk correction unit movably disposed at an end of the shaft thereof along the roll shaft.